2,6-Dimethyltetralin (hereinafter, referred to as “DMT”) is one of dialkyltetralins, herein the alkyl group is methyl, and a precursor of 2,6-dimethylnaphthalene (hereinafter, referred to as “DMN”). 2,6-DMN is a starting material to prepare 2,6-dimethylnaphthalate.
Industrially, 2,6-DMT is converted to 2,6-DMN by dehydrogenation in the presence of catalyst. The dehydrogenation process using the catalytic system, have been disclosed in U.S. Pat. No. 5,118,892, U.S. Pat. No. 5,189,234, U.S. Pat. No. 3,775,498 and U.S. Pat. No. 3,781,375. This reaction can be accomplished under gaseous or aqueous conditions. Preferably 2,6-DMT can be dehydrogenated in a gas state in the presence of catalyst at 600 ˜900° F. under 0.01˜25 bar at 0.1˜20/h of weight hourly space velocity.
Dimethyl 2,6-naphthalenedicarboxylate, that can be derived from 2,6-DMN, is a raw material of liquid crystal polymers, polyethylenenaphthalate (PEN) that is the precursor for highly functional polyester resins, and the like. Presently, several chemical companies retain its synthetic technique and a small number of company in the world has commercialized it.
Recently, PEN has attracted much attention as an engineering plastic of high performance for the next generation, since it has much better physical properties than those of PET that are used currently worldwide. In practice, the new products obtained from the PEN resins have the better crystal properties and the higher softening points than those from the commercially available PET resins. Besides, the PEN resins show much better performance in mechanical strength, thermal stability, resistance to chemicals, gas permeability, atmosphere corrosion resistibility, electrical insulation and the like.
Therefore, the demand for PEN will be enormous if a cost-effective process for the preparation of its starting material, 2,6-naphthalenedicarboxylic acid (hereinafter, referred to as “NDCA”), is developed and commercialized in a large scale. Concretely, PEN has potential applications for fast spinning fiber, 8 mm tapes and plastic bottles as a raw material, and for videotapes and special functional films as an end product.
The methods for preparing 2,6-NDCA have been already disclosed to those skilled in this art. Precisely, U.S. Pat. No. 3,856,855 has illustrated the process for preparation of NDCA comprising a step oxidizing DMN with molecular oxygen by using a co-catalyst system such as Co/Mn/Br in the presence of 4% wt or more of acetic acid per DMN % wt under 2˜8 bar of oxygen partial pressure at 100˜160° C. That is to say, 2,6-DMN is a major source material to manufacture 2,6-NDCA industrially at present.
In addition, Sikkenga et al. have demonstrated methods for preparation of 2,6-DMN in U.S. Pat. No. 5,073,670; U.S. Pat. No. 5,401,892; U.S. Pat. No. 5,118,892; U.S. Pat. No. 5,012,024; and U.S. Pat. No. 5,030,781 and so on. Concretely, the synthetic process composed of the multi-step reaction in liquid phase has been disclosed in those literatures. In that methods specific alkenyl benezene is cyclized to one or more specific DMTs in the presence of a proper catalyst of acidic solid, such as acidic crystal zeolite; are dehydrogenated to produce the corresponding DMNs; and then, the resulting DMNs are isomerized to obtain the specific DMN.
On the other hand, Thompson has illustrated the method for isomerization in U.S. Pat. No. 3,775,496, in which 5-(m-tolyl)-pent-2-ene is cyclized to 1,6-DMT and 1,8-DMT and then they are dehydrogenated to 1,6-DMN and 1,8-DMN and again the obtained DMNs are isomerized to 2,6-DMN and 2,7-DMN respectively. In addition, Thompson has disclosed in U.S. Pat. No. 3,775,498 that 5-(m-tolyl)-pent-2-ene is cyclized to 1,5-DMT; is dehydrogenated to 1,5-DMN; and then isomerized to 2,6-DMN.
Furthermore, Amoco (US) company has developed a process for preparation of 2,6-DMN comprising steps (1) adopting ortho-xylene as a starting material, alkenylation with 1,3-butadiene to prepare alkenyl benzenes; (2) cyclizing to obtain 1,5-DMT; then (4) dehydrogenation to prepare 1,5-DMN; and again (5) isomerization to 2,6-DMN and succeeded in business. Unfortunately, this process is complicated and problematic because a number of by-products such as 1,6-DMN are generated in the isomerization step to decrease the yield of the overall process (D. L. Sikkenga; I. C. Zaenger; G. S. Williams, U.S. Pat. No. 5,030,781 (1991): D. L. Sikkenga; I. C. Zaenger; G. S. Williams, U.S. Pat. No. 5,118,892 (1992): L. D. Lillwitz; A. M. Dkarachewski, U.S. Pat. No. 5,198,594 (1993)).
Also, Teijin (Japan) company has manufactured 2,6-DMN by alkylation or acylation of naphthalene or methylnaphthalene as a starting material. However, this process is not appropriate for production of 2,6-DMN in a large scale, due to the reaction efficiency, the lifetime of catalyst, the actual reaction conditions and the like (K. Sumitani; K. Shimada, Japanese Patent Application No. 1992-013637: K. Sumitani; K. Shimada, Japanese Patent Application No. 1992-112839: T. Fujita; K. Sumitani; K. Shimada, Japanese Patent Application No. 1992-049252).
In summary, the above-mentioned methods for preparation of 2,6-DMN, have mostly adopted 1-(o-, m-, or p-tolyl)pent-1 or -2-ene type of straight chained alkene compounds as the starting materials, and thus require the acidic catalysts for the cyclization.
Accordingly, after the dehydrogenation step, the reaction mixture contains different isomers of DMN, contaminants, by-products, remained DMT and alkenyl benezene as well as 2,6-DMN. Therefore, some extra steps of isomerization and separation are needed to obtain pure 2,6-DMN.
In order to obviate above-mentioned problems fundamentally, the present inventors have tried to exploit different starting materials and exclude the isomerization steps through different pathways of alkylation and cyclization in the process for preparation of 2,6-DMN, and consequently, have developed a novel process for highly selective preparation of 2,6-dialkyltetralin in a high yield.